Integrated circuit logic drivers and 3-state logic drivers using CMOS devices are well known. A logic driver circuit receives an input signal and provides an output signal that can be coupled to many other logic circuits. The purpose of the logic driver circuit is typically to drive a relatively large capacitive load, such as the load typically associated with the output pins of an integrated circuit.
A CMOS logic driver typically has an output stage that includes a PMOS transistor and an NMOS transistor coupled in series between two power sources. Ideally, only one of these two output transistors is on at any one time. Thus, when the input signal to a CMOS logic driver changes state, it is best if the PMOS and NMOS transistors switch in such as way that the two devices are never simultaneously on.
CMOS logic drivers are often implemented using integrated transistors which switch rapidly (often within 1-3 nanoseconds), even while driving capacitive loads. The harmonic content of the logic driver's output signal is a function of the rise and fall times of the output current signal. A smaller switching time represents a rapidly changing output current signal with many high frequency components, including harmonics.
Further, the high speed switching of the NMOS and PMOS output transistors can cause both of the output transistors to be on simultaneously during part of the transition time, thereby generating spikes in the circuit's supply current that aggravate harmonic generation. The resulting supply current signal has fast transition times, spikes and transients, and generally is rich in high frequency harmonics. These undesired harmonics radiate electromagnetic interference ("EMI") that can interfere with neighboring circuitry and indeed with circuitry external to the driver circuit.
Harmonic generation is difficult to control because CMOS parameters can vary fourfold with fabrication process tolerances, ambient temperature changes and power supply changes. While isolating EMI-generating CMOS devices from other circuitry may reduce the EMI problem, doing so is contrary to modern design philosophy. Integrated circuit designers today are encouraged to fabricate ever smaller, more densely packed and increasingly complex CMOS circuits within a given chip area.